The present invention relates to the field of organic sulphides and more particularly that of dimethyl disulphide.
Dimethyl disulphide (DMDS) has a strong and aggressive smell due in part to the presence of highly odorous impurities and in part to the garlicky and ethereal smell intrinsic to DMDS. This strong smell hinders the increased growth of this product in applications such as the sulphurization of catalysts or as loading additive for steam cracking. In comparison with other products used in these applications, such as tert-alkyl polysulphides, DMDS exhibits numerous advantages, in particular a high sulphur content (68%) and non-coking degradation products (CH4, H2S) Furthermore, in these applications, DMDS results in generally superior performances to other products, such as tert-alkyl polysulphides. However, these other products have markedly lower odorous levels which, for this reason, make them easier to handle in comparison with DMDS.
A particularly efficient and economical method, among the methods for the synthesis of DMDS, is the oxidation of methyl mercaptan by sulphur according to the reaction: 
This oxidation of methyl mercaptan by sulphur, catalysed by organic or inorganic, homogeneous or heterogeneous, basic agents under batchwise or continuous conditions, is accompanied by a release of hydrogen sulphide and of dimethyl polysulphides (CH3SxCH3) with a sulphur rank x of greater than 2. In order to manufacture DMDS according to this process with high yields and a limited production of DMPS (dimethyl polysulphides with a rank greater than 2), European Patent 0,446,109, the contents of which are incorporated here by reference, discloses a preparation process comprising two reaction regions interrupted by an intermediate degassing region and followed by a distillation region. Although giving a good performance in terms of yield and selectivity for DMDS, it turns out that this process results in a not insignificant amount of methyl mercaptan (approximately 4000 ppm) and a very small amount of dimethyl sulphide (approximately 300 ppm), originating from the methyl mercaptan used or produced during the synthesis of DMDS, being left in the finished product. The result of these volatile impurities is that they render the smell of the DMDS very unpleasant and aggressive and this strong smell is regarded as a significant cause of trouble during the handling of this product by users.
In order to mask the smell of organic polysulphides, U.S. Pat. No. 5,559,271 recommends adding a certain amount of masking product to them, such as, in particular, vanillin or ethyl vanillin. Although its general formula includes DMDS, this patent is more particularly targeted at the treatment of heavy polysulphides, such as, for example, di-t-nonyl pentasulphide. The application of this method to DMDS does not allow its nauseating and highly unpleasant smell to be masked.
It has now been found that, in the specific case of DMDS, the addition of an odour-masking agent is only effective if the DMDS used exhibits reduced contents of highly odorous volatile impurities, such as methyl mercaptan and dimethyl sulphide, and preferably comprises less than 200 ppm by weight of methyl mercaptan and less than 50 ppm by weight of dimethyl sulphide. It has also been found that the most effective odour-masking agents are not those mentioned in the abovementioned U.S. patent but those chosen from the esters corresponding to the general formula:
R1CO2R2 xe2x80x83xe2x80x83(I) 
in which R1 represents an optionally unsaturated, linear or branched hydrocarbon-comprising radical comprising from 1 to 4 carbon atoms and R2 represents an optionally unsaturated, linear, branched or cyclic hydrocarbon-comprising radical comprising from 2 to 8 carbon atoms.
The subject-matter of the invention is therefore a DMDS-based composition, characterized in that it comprises, by weight, at least 95% of dimethyl disulphide, less than 500 ppm of methyl mercaptan (MM), less than 100 ppm of dimethyl sulphide (DMS) and up to 1% of at least one odour-masking agent, preferably an ester of general formula (I).
Any method known to a person skilled in the art for producing a DMDS with reduced contents of volatile impurities, such as MM and DMS, can be used in the context of the present invention. However, in the case of a DMDS comprising high contents of MM and DMS, a particularly preferred method consists of a distillative topping. This method exhibits the advantage of jointly removing MM and DMS, whereas the usual methods for smell reduction, generally based on removing residual mercaptans by specific reaction of a mercaptan functional group with a removal agent, such as a base or an alkene oxide in the presence of a base, are without effect on DMS present in DMDS.
The DMDS, thus topped, which preferably comprises less than 200 ppm of MM and less than 50 ppm of DMS, is used to prepare a composition according to the invention by simple addition of at least one odour-masking agent.
As one of the main advantages of DMDS in its applications is its high sulphur content (68%), an excessively high content of odour-masking agent in the composition would result in this sulphur assay being lowered and would decrease the advantage of this product in its main applications. The maximum content of odour-masking agent(s) is therefore set at 1% but this content is preferably between 0.1 and 0.5% and more particularly equal to approximately 0.2%.
Mention may be made, as illustrative but non-limiting examples of esters of general formula (I), of butyl acetate, isoamyl acetate, benzyl acetate, ethyl butyrate, propyl butyrate, butyl butyrate, 2-methylbutyl butyrate or isoamyl butyrate. Isoamyl acetate, 2-methylbutyl butyrate, isoamyl butyrate, benzyl acetate and the mixtures of these compounds are more particularly preferred. The esters (I) may or may not be used in combination with ortho-phthalates corresponding to the general formula: 
in which the symbols R3 and R4, which are identical or different, each represent an optionally unsaturated, linear, branched or cyclic hydrocarbon-comprising radical comprising from 1 to 8 carbon atoms. Mention may more particularly be made, as non-limiting example of compound (II), of diethyl ortho-phthalate.
A typical composition of the present invention comprises, by weight:
Another typical composition of the present invention comprises, by weight: